package niu;

import java.util.Scanner;
public class ListNode {
    int val;
      ListNode next;
      ListNode() {}
     ListNode(int val) { this.val = val; }
      ListNode(int val, ListNode next) { this.val = val; this.next = next; }
  }
    class Solution {
        public ListNode mergeTwoLists(ListNode l1, ListNode l2) {

            if (l1 == null) {
                return l2;
            }
            if (l2 == null) {
                return l1;
            }
            ListNode dummyHead = new ListNode(-1);
            ListNode last = dummyHead;
            while (l1 != null && l2 != null) {
                if (l1.val <= l2.val) {
                    last.next = l1;
                    last = l1;
                    l1 = l1.next;
                } else {
                    last.next = l2;
                    last = l2;
                    l2 = l2.next;
                }
            }
            if (l1 == null) {
                last.next = l2;
            }
            if (l2 == null) {
                last.next = l1;
            }
            return dummyHead.next;
        }
    }

